This invention relates to a method of controlling a grinding process for a cylindrical or internal grinder, in which the changing point from the rough grinding to the fine grinding is automatically shifted, particularly, to a controlling method to adjust or shift the changing point (change-over size point) during response to the metal removal rate of the workpiece in the rough grinding.
In the grinding method having a rough grinding step and a fine grinding step, there is, for example, residual stock removal controlling method as follows.
In the residual stock removal controlling method, both the infeeding amount of the infeed table and an actual workpiece size are measured during working, successively detecting the residual stock removal or difference between the infeeding amount and the size to control the infeed speed keeping the residual stock removal constant. An example of a working cycle of this method is shown in FIG. 1, in which the solid line shows the infeeding amount and the dotted line shows the workpiece size. At first, the infeed table is fed rapid till the grinding wheel is positioned to work on the workpiece. At this beginning state, residual stock removal begins to occur and increases till a first predetermined residual stock removal d.sub.1 for rough grinding, the rapid feeding finishing there (stop A). After reaching at the predetermined value d.sub.1, the infeed table speed is controlled so that residual stock removal is kept at d.sub.1, with working on the workpiece proceeding (rough grinding step B). When the workpiece size reaches at a predetermined changing point R.sub.1 from rough grinding to fine grinding, the infeed table speed control is changed to keep a second predetermined residual stock removal d.sub.2 (&lt;d.sub.1) by a change-over signal. At this change-over point, the infeed table is kept over-fed due to large residual stock removal of rough grinding so that it must be quickly retracted till residual stock removal decreases to be equal to a second predetermined value d.sub.2 (step C) and after that the infeed table is fed so as to maintain residual stock removal value d.sub.2 (step D). Workpiece size reaching at the finish size R.sub.0, the infeed table is returned quickly to its original location and one grinding cycle is accomplished.
In this residual stock removal control method, the change from rough grinding step B, during which a first residual stock removal d.sub.1 is kept, to fine grinding D, during which a second residual stock removal d.sub.2 is kept, is carried out when workpiece size reaches at a predetermined value R.sub.1. And there is naturally some time lag or response time (e.g. 0.1 sec.) from the electric changing signal generation till the moment when the residual stock removal decreases till the second residual stock removal d.sub.2.
During this time lag, grinding of the workpiece is, off course, continued and the workpiece size is changing. The grinding amount in this transient time is larger when the grinding ability is good and vice versa when it is poor. Accordingly, good grinding ability decreases the stock removal for fine grinding so that the finish size is attained in a shorter time, and poor grinding ability increases the stock removal for fine grinding so that it requires a longer time to attain the finish size. That is, the deviation of the grinding amount in the aforementioned transient time causes deviations of working accuracy (particularly cylindricity, surface roughness etc.) and the cycle time (working time).
In other grinding methods in which rough grinding infeed speed is changed at a proper changing point to fine grinding infeed speed, there also occurs the aforementioned disadvantage.